Synthetic petrochemical-based polymers have had an industrial impact since the 1940s. Despite the numerous advantages of these materials, two major drawbacks remain to be solved, namely, the use of nonrenewable resources for their production and the ultimate disposal of these large-scale commodity polymers. Due to their unique properties, biodegradable polymers have been considered as alternative environmentally friendly polymers, and the advances achieved over the last 30 years in the synthesis, manufacture, and processing of these materials have given rise to a broad range of practical applications from packaging to more sophisticated biomedical devices. Of the variety of biodegradable polymers known, linear aliphatic polyesters are attractive and most used. Notably, these polymers are not only biodegradable (the aliphatic polyester backbone is intrinsically sensitive to water and heat) but also bioassimilable, since their hydrolysis in physiological media gives nontoxic components that are eliminated via the Krebs cycle as water and carbon dioxide.
One of many methods in synthesizing these polymers is the ring opening polymerization of the corresponding cyclic lactone monomers or lactide (LA). Many catalyst systems have been evaluated for the polymerization of lactide and lactones including complexes of aluminum, zinc, tin, and lanthanides. Even strong bases such as metal alkoxides have been used with some success. Tin compounds, especially tin(II) bis-2-ethylhexanoic acid (tin octoate), are used for bulk polymerization due to their solubility in molten state, high catalytic activity, and low rate of racemization of the polymer. Conversions of >90% and less than 1% racemization can be obtained while providing polymer with high molecular weight. The polymerization of lactide and lactones using tin octoate is generally thought to occur via a coordination-insertion mechanism. High molecular weight polymer, good reaction rate, and low levels of racemization are observed with tin octoate catalyzed polymerization. Typical conditions for polymerization are temperatures in a range of 180±210° C., catalyst tin octoate concentrations in a range of 100±1000 ppm, and reaction times in a range of 2±5 hours to reach about 95% conversion. The polymerization is first order in both catalyst and monomer. Frequently hydroxyl-containing initiators such as 1-octanol are used to control molecular weight as well as to accelerate the reaction. One issue in commercialization is the catalyst residue. In spite of the versatile applications of Lewis acids in organic synthesis, their use in polymer chemistry has been quite limited. Certain catalysts have been used, but a major hurdle regarding the commercialization of such processes using certain catalysts is the difficulty in removing catalyst residues and the cytotoxicity associated with such residues, which limit the use of these polymers in biomedical applications.
Accordingly, there is a need for new catalysts that have environmentally benign metals that are constituents in the mammalian anatomy so that the residues are harmless. The embodiments described herein fulfill this need as well as others. The embodiments described herein in are based upon, in part, the synthesis of poly(lactide) (PLA) using mild Lewis acids as catalysts.